The present invention relates to a core construction for a boiling water nuclear reactor.
A core of a boiling water reactor is constructed by arranging a multiplicity of core units 14. As shown in FIG. 1, the core unit 14 consists of four fuel assemblies 12 which are arranged in a square adjacent a control rod 13 at their center. Each of the fuel assemblies 12 comprises a plurality of fuel rods 11.
Generally, the nuclear reactor is continuously operated in a planned term, and the term is called a cycle. After the nuclear reactor is operated in the cycle, some fuel assemblies are replaced by new ones, and the core construction of the nuclear reactor is rearranged.
Then the nuclear reactor is continuously reoperated in the next cycle.
Generally, 1/4 or 1/3 of the total fuel assemblies are replaced by new fuel assemblies at the start of a cycle. Such core construction is called four or three batch replacing core, respectively. The new fuel assembly newly loaded in a core is called "the fuel assembly of first-burning cyclic period". The fuel assembly burned up in one cycle and remained in the core in the next cycle as its second cycle is called "the fuel assembly of second-burning cyclic period". The fuel assembly burned up in two cycles and remained in the core in the next cycle as its third cycle is called "the fuel assembly of third-burning cyclic period". The fuel assembly burned up in three cycles and remained in the core in the next cycle as its fourth cycle is called "the fuel assembly of fourth-burning cyclic period".
The control cell is a core unit in which a control rod is adjusted during normal operation of the reactor.
In recent years, fuel assemblies of higher enrichment are being used, and core construction of more than four batch replacing core is considered. In this type of core construction, however, thermal power densities of the fuel assemblies of the first- and second-burning cyclic period become higher, the power distribution in the core becomes less uniform and sub-critical margin decreases.
Japanese Patent Disclosure (kokai)56-129888 discloses a core construction of more than four batch replacing. The core has a symmetry of rotation of 90 degrees, and more than 60% of the total core units include each different burned fuel assembly. However, each of the 1/4 cores is not symmetric itself.